Avoid duplicit records in mcp_salt and mcp_extra

Change-Id: Ib30e8d1e18552b4b9733743db946091b4baf917d
diff --git a/linux/system/repo/mcp/extra.yml b/linux/system/repo/mcp/extra.yml
index 0bd9f13..d8977f4 100644
--- a/linux/system/repo/mcp/extra.yml
+++ b/linux/system/repo/mcp/extra.yml
@@ -8,3 +8,4 @@
           source: "deb [arch=amd64] http://apt-mk.mirantis.com/${_param:linux_system_codename}/ ${_param:apt_mk_version} extra"
           architectures: amd64
           key_url: "http://apt-mk.mirantis.com/public.gpg"
+          clean_file: true